Many people are still waiting for a official new Jailbreak application to Jailbreak the latest and greatest creation from Apple. iOS 4.0 Gold Master Candidate. BUT! There is a way! You don’t need a Dev account to install the Gold Master!
The post is only for iPhone 3G users and people who own a Macintosh! For iPhone 3GS users, it's here.

1 (very important, related to Cydia)
The first you’ll start Cydia it will take some time to load. Do not interrupt this proces!! If you’ll do this, some applications might stop working and can dissapear from your phone! Just make sure to let Cydia do it’s own thing, and don’t let the phone go into sleep mode!

2 (very important, related to Cydia)
It might be possible that after you’ll start Cydia, it doesn’t have any sources. Please add the following source http://cydia.xsellize.com/. Cydia will give some errors, just ignore them! When it’s done Cydia asks you to upgrade Cydia. Choose “Complete Upgrade”. Wait for it to finish. After this process, Cydia should have all it’s sources back and the white icon on your springboard is also fixed!

3(syncing)
It is not possible to sync your iPhone with Windows or with iTunes 9.1.1 after you’ve upgraded to iOS 4.

4 (safari crash)
Some people experience that the Mobile Safari app keeps crashing after the jailbreak. It turned out that I had a link to Redsn0w 0.9.5b4 instead of 0.9.5b4-2. I’ve updated the link. So if you’re having any trouble with Safari, please check if you have the latest version of Redsn0w!

5(battery life)
Some people wonder if the new firmware (and multitasking) affect battery life. This is not the case. Apple did a great job implementing multitasking.

NOTE : I’m not responsible for any damage to your device! All thanks go to Redsn0w and other people who work hard everytime for us, to jailbreak our devices.